Electric siren



March f7, 1950 Y w. o. CRANE ELECTRIC SIREN Filed Nov. 26, 1945 22 219 Ww 3/ 2 057 l l 60 25- @62 ff f7 H n ,r fg Il j AMP. fm J2 as -M//yw--Ikl fvr;

March 7, 1950 w. o. CRANE 2,500,053

ELECTRIC SIREN Filed Nov. ze, 1945" '2 sheets-sheet 2 .3. E35 Q ma. QN @Elow 3 2 m.

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@Hell/(02? /eseff 0 Crane f5 I' l J RM @www patented Mar. i7, 1954() UNITED STATES raTENT oI-FICE ascuas?. nLEo'rnio sinEN Webster Crane, St. Charles, Ill., assignor to Operadio Manufacturing Co., St. Charles, Ill., a corpora-tion of Illinois Application November 26, i945, Serial No. ,630,928

Claims.

quency oscillator is provided, which oscillator if utilizes the non-linear characteristics of vacuum tubes and also iron-core transformers. In addition thereto, circuits are provided in which relatively slow transient effects are utilized for controlling the oscillator. By utilizing transient I phenomena, a slow variation of potential availships and war vessels have loud speakers as part i of` public address systems. It is highly desirable to provide means for utilizing such loud speakers to provide siren tones in times of danger, as warning signals or the like.

ln order to obtain a true siren effect, it is necessary that a substantial frequency variation of emitted sound be provided, that such variation occur at a relatively slow rate and that the intensity of sound appear to increase with increase in pitch. The frequency variation itself should be continuous from a minimum value to a maximum Value and back again, A complete frequency variation cycle, hereinafter referred to as a siren cycle, is relatively slow and may have able in an oscillator may be provided. While either the transient eifect and the non-linear reaction may be relied upon independently for generating` sound waves as warning signals, it is preferred to combi-ne the two in a siren.

The invention will now be described in connection with the drawings wherein exemplary embodiments are disclosed, it being understood that this is merely illustrative and may be modified within wide limits. In the drawings, Figure l is a duration of the order of from about one second frequency will not sound as loud and will not be as piercing as at a higher frequency. However, the sound energy with increase in frequency should not decrease to the point where the siren effect is lost. Preferably, the actual power output should increase with frequency.

While various means for obtaining different frequencies are known, a simple and effective generator of siren current waves has hitherto not been available. Inasrnuch as sirens are used generally in times of danger, it is essential that the system as a whole be fool-proof, dependable and be simple. From the point of View of cost and service, it is desirable that a system of this character have a minimum of components and use standard parts as much as possible.

In accordance with the invention, there is provided a system which is simple, has relatively few components, substantially all of which are standard components in audio and radio frequency work. A system embodying the present invention is not critical, requires no critical adjustments, and may be easily adjusted to suit different requirements with regard to pitch, volume variation and performance in general.

In accordance with the invention, an audio frea circuit diagram of a siren system. Figure 2 shows some characteristic curves for the system of Figure 1. Figure 3 is a circuit diagram of a warning system wherein the frequency is fixed.

Referring to Figure l, vacuum tube IIJ `has cathode l l connected to ground through bias resistor l2. Bias resistor l2 may be shunted by by-pass condenser I3 to prevent cathode degeneration. Vacuum tube l 0 has anode i5 connected to lead i6 going to primary Il of transformer I8. Transformer I8 is of the iron-core type and preferably is so designed that the iron core readily saturates. The core is preferably of Mumetal or other ferromagnetic metal having a sharp knee on the B-I-I curve. Primary l1 is connected through lead 26 to junction point 2|. From junction point 2|, a connection goes to resistors 22 to 24 inclusive, all in series, and thence to junction point 26 for connection to a source of B plus voltage. Grounded condensers 21 and 28 are connected to junction points 29 and 30 respectively between resistors 22 and 23 on the one hand and 23 and 24 on the other hand. From junction point 3l between resistors 23: and 24,

` lead 32 is connected to dropping resistor 33,

thence to junction 34. Junction 34 has by-pass condenser :it connected to ground, this condenser having substantial capacitance. Junction 34 is connected to cathode Il through resistor 31.

Going back to vacuum tube I0, control grid 4D is connected to junction point 4 l. Junction point iii is connected to ground through variable resistor d2 and is also connected by resistors 43 and 44 in seriesto junction 2l. Junction 45, between resistors 43 and 44, is connected to grounded icy-pass condenser 46, this condenser having substantial capacitance.

Transformer I8 has secondary 48 with. one terminal grounded, and the other terminal connected to junction 49. Junction 49 has resistor terminal of this condenser being connected toV junction 55. Resistor S is connected between junction 55 and ground. Junction 55 forms an output terminal ior connection toamplier, '51

with the output of the amplifierv going to speaker 58. Speaker 50 is preferably a loud speaker having sufficient power handling. ability to provide proper siren effects.

Shunted across resistor 2li is switch Se which maybe opened and closed by any suitable means. A switch opening and closing cyclewill determine a-.Siren cy-cle so that the opening and closing of switch 60 mayv beicarried on ata relatively slow rate. Thus, for automatic operation, switch 60 may be opened andclosedby cam 6il driven from motor 62; Cam 6I has rise 63 extending for l180 degrees so that, for one revolution of cam 6I, switch l is closed half of the time and open half the time. Cam 5 Itmay make about lteen revolutions per vminute so that switch 60 will be closed for two seconds andv open for two seconds. It is not necessary that a siren cycle be divided into equal parts. It is understood, however, that the time of switch operation may-oe varied and other means for shorting out resistance 24 may be provided. Thus, resistance 2li may be the plate to cathode resistance of an electron discharge tube. Switch 50 may-be replaced by bias control means on a control grid. When the grid potential of an electron kdischarge tube is positive or about equal to that oi the cathode, the plateto cathode resistance will be low and may be considered negligible for all ordinary purposes. Thiswould correspond to a closed switch condition. With,

thegridbias made negative to the cathode but above-cutv off, the anode to cathode resistance may b e-adjustedto any desired'high value.

Resistance 2li is preferably largein comparison to resistors 22 and 23. Condensers 21 and 28 are also large and ofthe order of filter condensers in the power supply of an average radio receiver. Resistors 43 and 44 are preferably high enough so that, together with resistance 42, the bias of control grid' lrnay be adjusted to a point above out oli. Transformer I8 has a step down ratio of the order of ten to one, it being understood that the secondary voltage is about one-tenth of the primary.

In.A a typical system, the various elements have the following values:

Transformer I8 has a laminated core of 4 Mumetal. Primary I1 has an inductance of 7.5 henries at 1000 cycles per second with a primary to secondary ratio of ten to one. The transformer is so designed that the inductance change shall be at least 80% when measured under the conditions of zeroD. C. in the. primaryI and 3 milli-amperes in` the primary and computed by dividing the difference between the two inductance values by the inductance at 1.85 milliamperes. The connections of the primary and secondary are so chosen that, when a voltage is applied acrossprimary I'I, the potential between the primary-terminal connected to line i6 and the secondary terminal connected to ground should be the sum of the primary and secondary voltages. The primary has 3610 turns of number 40 wire, while the secondary has 361 turns of number 36 wire. The core is a shell type with F laminations. The stack is, high andmeasures 11% x 1%;7". Thewindow is %j" X 1A.

To energize thek above system, a voltage ofr 300 was applied between terminal. 20. and ground.

Referring4 now to Figure 2, some characteristic curves, in connection, with theftypical system of` Figure. 1 is given. Thel curves in full lines are' drawn to a voltage scale shown at the left, while the curves in dotted lines aredrawn tothe scale shown at the right. The horizontalgaxis Shows time in seconds with switch closed at zero time. and opened atI around 1.67 seconds after zero time. The numbers onthecurves show the volt-` age with respect to ground of. the points in the circuit bearingy the corresponding` numbers.

Thus, curveV 3A,y shows the comparatively sharp rise in voltage upony closureoiswitch 60. Curve 29 shows the rise involtage at pointl 2.9, this being with` respectto ground as previously indicated; Curve ZI shows thevoltagerise at point 2 I. This shows the additional iiltering action of the varioust condensers. Curve I6` shows, `the voltage. at the anode of vacuum tube ID. All the preceeding curves show direct current voltages.

Curve I6a showsv the envelope ofy alternating potential at anode I5. It isr understood that the actual potential oscillates around the zero potential line with the` frequency of oscillations varying between the limits vpreviously given.

The remaining curves and solid lines crowded near the zero axis are merely shown to the same scale as the preceeding curves for purposes of ready comparison. These curves have been reproduced. to, a larger scalev and are shown in` Thus, curve 45' shows the direct` dotted lines. current potential at point 45. Curve-55a shows the envelope' ofthe alternating potential at the point bearingthe corresponding number. Curve dlal shows the envelope of the alternating current potentialv at point- 4.I which, in effect, is control grid` 40. Curve 49a givesthe alternating potential envelope at: the high side of transformer secondary 48, Curve` II showsthe directcurrent potential at the cathode of vacuum tube I0, while curve IIa shows the envelope of the cor respondingalternating potential. Curve 4I shows the dire-ct current potential at the grid input. CurverP shows the` power output in decibels at terminal 55. The scalev at the right is` used for this curve, although it is understood that the scale does not refer to volts but is merely an abstract number.

At the openingA ofl the switch;A they decay of the various curvesl is clearly indicated. It is clear from the. curves. that the risev in absolute f value at the'timev of the closureof. the switch of the various. voltages isjsomewhat, faster than the decay of these same voltages at the opening of the switch. It will be noted from curves 45 and 4I that the relative bias potential of the control grid with respect to the cathode drops in an algebraic sense. Thus, just prior to switch closure, the control grid is biased to about two volts negative to the cathode. Just prior to the opening of the switch, the grid potential is about 12 volts negative with respect to the cathode.

In general, this change in grid bias is due to the increase in amplitude of oscillations at the secondary of transformer I8. It is clear that, on a positive swing of control grid 4Q, a comparatively low resistance discharge path through the vacuum tube to ground will be provided for condenser 5?. On the other hand, on the negative swing of control grid 40, the time constant of the discharge path of condenser 52 will be increased. Thus, after closure of the switch, the oscillations generated by vacuum tube IB in conjunction with transformer I8 will be similar to that of a blocking oscillator and gradually charge condenser 52 so that grid llo becomes increasingly negative.

It is clear, however, from the shape of the various curves that, unlike a blocking oscillator, the system will continue oscillating indefinitely with the switch either open or closed. With the switch closed, the amplitude of oscillations builds up, thus increasing the power output of the system. The increased current iiow through the primary of transformer I8 will decrease the inductance of the transformer and thus tend `to raise the natural frequency of oscillations. The closure of switch Bil results in higher potentials being impressed on vacuum tube I0. As a ren sult, the amplifying factor of the vacuum tube is changed, and the amplitude of oscillations is increased.

Upon opening of switch 6D, a reverse action ensues, and conditions return to those existing just prior to the time when the switch was closed. Due to 'the decreased potential and the discharge of the various condensers, transformer I8 tends to reduce the natural frequency of the system. At the same time, the amplitude of oscillations in vacuum tube lil decreases. As will be apparent from curve P, the decrease in power at the output of the system is slower than the rise in power when the switch is rst closed. Thus, the Volume of sound of the siren increases with increase in pitch, the increase being faster than the decrease.

While the values of the various components may be varied over wide limits, it will be 0bserved that the siren provides two substantially different conditions in an oscillator. Under one condition, when the pitch and volume are at a minimum, the applied voltage at the system is comparatively low, resulting in maximum inductance at the feedback transformer and minimum voltage amplitude. When power and frequency are a maximum, the applied voltage to the system is a maximum with inductance at the feedback transformer reduced to a minimum and the amplitude of oscillations at a maximum. In general, while the various components may be adjusted to various values, it is preferred to have the time constants of the cathode, grid and anode circuits all about the same value and substantially less than the period of a complete siren cycle. As shown here, the time constant of the various cathode, anode and grid circuits are of the order of about one-tenth of the period of a siren cycle. This may be varied to soy tive and negative swings.

be somewhat larger or smaller, depending upon desired operating conditions.

Best results in approximating a siren tone have been obtained by so designing the system that the output energy, as indicated by curve P, has as much of a linear rise and drop as possible. As will be noted from the curves, the decay is more linear than the rise. Also, it is preferred to have the variation of power with respect to time greater on the rise than on the fall. However, the variation of power and pitch should be slow enough to be audibly sensible. In other words, the slope of the rise on curve P is substantially greater than the slope of the fall of this curve after the switch is opened. A linear rate of the logarithm of power variation appears to be desirable. Thus, a db.` curve should have a generally linear rise and fall. The purpose oi this is to simulate :a motordriven siren. On the speed-up of the siren, the

motor increases its speed faster than on the re-` verse portion of the cycle where the mechanical system would tend to coast.

It it understood that the saturating effects present in transformer I8 and the disposition of the secondary in the grid input circuit, together with transient effects, will tend to prevent the generation by vacuum tube l0 of any p-ure sine wave. In other words, the system does not generate oscillations having a sinusoidal wave shape at the resonant frequency. It has been found that the entire system when operating is rich in harmonics as is true in over-excited oscillators. This is particularly true of the clipping action in the grid circuit on posi- In particular, condenser 54 and resistance 58 may be considered as comprising a filter section for emphasizing a harmonic. In the example given, the values are such that a third harmonic is particularly emphasized.

As a rule, only odd harmonics are present in the output circuit. This is believed to be due to grid clipping on both positive and negative swings. When the grid goes positive, the low grid to cathode resistance effectively shorts resistance 42. The entire circuit involving the secondary of transformer I8 has the frequency determining constants changed sharply. Dur; ing the part of the cycle when the grid is positive, transformer I 8 is believed to saturate.

This is particularly true when switch 6E Iisclosed, impressing a high voltage on both the transformer primary and vacuum tube. The saturation of transformer I3 occurs over a larger part of `a cycle under these high voltage conditions, Thus, the transformer and oscillator both introduce distortion during a part of each cycle of oscillation.

In the system given above, the frequency when switch 60 is closed is around 1200 cycles per second and, when the switch is opened, the frequency drops to around 600 cycles per second. By variation of resistance y42, the range of frequency between the maximum and minimum may be controlled. The value of condenser 52 has an effect on the resonant frequency.

It is evident that switch 50 may be retained in either open or closed position and the entire system be energized or deenergized for use as the generator of a simple constant frequency warning signal. However, where a single output frequency is desired to simulate a motor-driven horn rather than siren, the circuit may be simplifled somewhat as shown in Figure 3.

Itgeferrngv how to -Figure 3, vacuum-,tuberl has cathode 16 grounded; while anode-Wis connected to.1ead11il.3 Lead 'lpisoonnected to terminal'` 'le offfprimary Siluof iron core'transormer di. Primary ghascondenser 3,2shunted across. Tenminal' 83 of primary isconnected by lead Btl to. resistance 85: Resistance 8,5, is connected through junction S'to resistance gland thence, through switch St,V to E plus` terminal 90. Grounded condenser S'i is.Y connected to juncftion 8S,

Transformer 8i, hassecondary 03 shunted by condenser 94, one terminal o the secondarybe- :ing grounded at S5, while terminalis connected 'through resistance di to junction4 QS. Junction I98,is,c,onnected through condenser SQ: to junctiony |00, Grounded resistors lili and 56E; are connected toA junction its., and ldll retlectively. Vacuumitube 1,5has control grid Ulliv connected toh-,jurictionl 00, this junction also being connected; through resistance 05 to junction-litt.; Junction 10,6 has resistors itlv and; 03 con nected on each side thereof andy thus for-ms a potential dividing network. Resistance ll is grounded, while resistance 08Y is connected` to terminal H0. Terminal iid anad ground.v are adapted to lb e supplied a 6% cycle source of alternating, current.

Lead lgvisconnected by condenser itil-fand resistance H31-to` output terminal lill. Resistance is connected between ground and the output terminal.v

Transformer 3l may have a step-down rat-io substantially the same as that of Figure l. Since the` system operates at a fixed frequenc any transforme1'having theinductance for determining.v a desired frequency may be used.

Inepractice, load resistors dit and. Si need not havera veryfhighvalue and maybe ofy theorder of between 5000 and 10,090 ohms each. Condenser 9i preferably has substantial capacitance of theYorder-of about .-m. i. Condenser may be. Variable and, in practice, may be any one of a, number of fixedV condensers controlled by a suitable switch. Condenser 8,2,v has a comparatively smallcapacitance ofthe order of .0l ni. f. Condenser Qll ony the other hand has a higher capacitance of the order or .l m. f. f Resistance 9J 'is not very large as a rule, being somewhat over 5000 ohms. Condenser limay have a capacitance o f the` order ofy 0.5, while resistance I'Uvlv and |02 may each be about one-half megohin. Resistance H05 is quite high., of the order of about one,megohnnwhileresistance lil' andv lil@ may besmaller such as about 300,000 ohms each. The actual ratioof valuesof itil, and E03 will be determined byy the operating bias onr control grid |04.

Vacuum tube l5 may be type SSNI. Condenser l I2 isfairly small such as 05m. f., while resist-v ance` l'l,3.may be of the order ofA 50,000cr 75,000 ohms. Resistor l t5 may be oi the order of 16,000 o r 12,000 ohms.

With a BV supply of about 300 volts and a 60 cycizesignalirof abouty one volt on. the tube grid,

' afsystem having the values previously given will oscillate between about 40)` and 500 cycles per second.) The cycle signal or any other suitable signall merely provides noise background, The output lter consisting of condenser HZ andresistances l .l 3,y and I l5, in this particularinstance, tends to pass theseventh harmonic. An amplia @gaand speaker may be provided for utilizing the. output voltages,

The output from, terminal #inlay be fed` into of. Figures- 1 and 3 that positive grid` clipping is greater than negative grid clipping. In Figure 3, they transformer`v primary circuit is resonant top aderlnite frequency, Whilethe secondary cir- @uit has its Constantsalteredibyf viriueof the grid going positiveandshunting out resistor l02. Inbothl systems, a formV of r frequency modular tion during parto f eachcycle vo the-'fundamentaloccursi Itisclear that, by controlling the various distortion`v cliaracteristics,l any desired harmonic, usually odd, may be particularly, ernphasized.

In general, however, the acoustic output from a speaker may be used toV determine thel sys@ temi Constants, ei'dtly. nl?. hamo; @nal-V515 orf-distorted outputsis-difculttodperform. Where a sirven teneisV desired, the exact eiectsmay-be duplicated by controlling-the various RCcircuits and listening to thespeaker-output. In general, however, the curves in Figure Zwillvbe a guide.

Thus, the inventionY provides an over-excited Oscillator including a Seturabletraosformer RC circuits are provided to vary-the degree of overexcitation.v Inthesiren, the RC circuitsfor the tube electrodes all have substantially similar tirne constants undervoperating4 conditions. Thus, one circuit is bound to affect-another circuit 4because of coupling.

It is understood that only during thelpositive portion of a cycle. (on-the control grid) willthe transformer, saturate -if it saturatesat all. When switch 60, is open,'the current thro-ugh the primary drops sothat the transformer core Works well below theJneeof tlBHl-Ilr curve as far as the direct ourrentcomponent isconcerned. After the switch has been closed,4 theI direct current component ,mayy be` sufcientksto saturate the core. Of course, thealternating currents superimposed onthe directlcurrent componenty may cause the transformer corerto go above or below, the direct current operating point, Thus, the oscillator is always over-excited to provide harmonics. rThe fundamental is altered by movingtheoperating point ofthe transformer core, this being the point on the lt-Hrcurve due tothe D. C. component. In practice, the effective inductance` of the transformer need notnecessarily vary. As a result of grid clipping amplitude modulation ofthe output occurs. This', together-with frequency variation withchange iny potential, results inwarbling.

The multi-section lter to the primary tends to linearize. the `voltage rise, ordecay with change in conditionoftheswitch, ByL careful control of the relationship between time yconstants in the anode, control gridland cathode Circuits, different operating chara cteristics may be obtained. rlhus, the instantaneous operating point of vacuum tube it may be variedl due to difference in voltage variation between the potentials on the various tube electrodes. The over-all character.- istics of theoscillator may bevaried by controlling the relation between time constants in the various tube electrode circuits. In practice, the diierencesbetween.time constants willbe small compared to a siren cycle. The reason is that the instantaneous operating characteristics of the system cannot change too fast or the ear will not detect the change in over-all output characteristics. i

What is claimed is:

1. An audio frequency oscillator comprising a transformer having primary and secondary windings, a vacuum tube having cathode, grid and anode electrodes, a connection between said anode and one terminal of said primary, load resistors connected to the other terminal of said primary, at least one condenser of substantial capacitance connected between ground and said load resistors, the free terminal of said load resistors constituting terminals for connection to a source of direct current at high potential, connections disposing said secondary between said cathode and grid so that continuous oscillations may be generated, a voltage divider network connected between a load resistor and ground, Said voltage divider network having an intermediate point thereon connected to said control grid and means for slowly switching a load resistor in and out of circuit, said load resistors and condensers together having a time constant about one tenth v that of a switching cycle.

2. An audio frequency generator of siren tones said primary and said control grid, a condenserv shunting part of said last-named resistor, a resistance connected between the terminal remote ground, a condenser and resistance connected comprising a vacuum tube having cathode, grid y and anode, a transformer havingprimary and secondary windings with a step-down ratio, said secondary winding being connected across said grid and cathode, a connection from one terminal of said primary to said anode, a resistance condenser filter connected to the other terminal of said primary, said resistance condenser filter having terminals for connection to a source of direct current at high potential, means for impressing two substantially diierent voltages on said lter successively at regular intervals to provide a siren cycle, saidl filter having a time constant about one tenth that of a siren cycle so that the potential impressed upon said primary varies, said transformer windings being poled regeneratively as to maintain the vacuum tube continuously in oscillation, and an output circuit `connected at the anode of said vacuum tube.

3. An audio frequency oscillator for a siren comprising a vacuum tube having cathode, control grid and anode, a grounded resistor connected to said cathode, a first condenser across said grounded resistor, a resistance and condenser in series between said cathode and ground, an iron core transformer having a saturable core under normal operating conditions, said transformer having a secondary providing a step-down ratio a third condenser connected between the control grid of said vacuum tube Aand one terminal of said secondary, the other terminal of said secondary being grounded, grounded resistors connected on opposite sides of said'third condenser, a connection from one terminal of the primary of said transformer to said anode, a connection from the other terminal of said primary to a plurality of resistors in series and thence to a B plus terminal, said B plus terminal being adapted to be supplied with direct current at high voltage, the resistor adjacent the B plus terminal having a high value so that when it is snorted out the voltage at the anode of said vacuum tube may vary through'a substantial range, means for cutting in and cutting out said last-named resistor adjacent the B plus terminal, grounded condensers connected between the junctions of `said remainingV resistors in series to provide a multi-section lter, a resistor connected between said other terminal of between the anode and ground, said oscillator having output terminals across said last-named resistor.

4. An audio frequency generatoi` of waves for a siren system comprising a vacuum tube having cathode, control grid and anode, a biasing resistance connected between ground and cathode, a first condenser shunting said biasing resistance, a resistance and condenser in series between said cathode and ground, an iron core transformer having primary and secondary windings and having a step-down ratio, said secondary winding being connected between ground and one terminal of a second condenser, a connection between the control grid and other terminal of the second condenser, grounded resistors connected on oppositesides of said second condenser, a resistance connected to one terminal of said transformer primary and to a grounded condenser, a connection between said control grid and the high side of said last-named condenser, a multi-section resistance and condenser iilter connected to said one terminal of said transformer primary, a dropping resistor connected between said multi-section filter and a B plus terminal, said B plus terminal being adapted to be connected to a source of direct current at high potential, a connection between said multi-section filter input and the junction of said resistance and condenser in series between cathode and ground, a connection between the other terminal of said primary and said anode, a condenserand resistance connected in series between said anode and ground, and an output circuit connected across said last-named resistance.

5. An audio frequency oscillator comprising a three-electrode vacuum tube having cathode, grid and anode electrodes, an iron core type of trans-` former having primary and secondary windings, connections disposing said primary in the anode circuit and said secondary in the grid circuit, said windings being regeneratively poled to maintain continuous oscillations, resistance-condenser circuits in the cathode, grid and anode circuits, said circuits having generally similar time constants of the order of between .1 and .4 second and means for impressing two diierent potentials on said system at successive intervals, said system oscillating at audio frequencies to produce a siren effect with a siren period of the order` of about two seconds. l

WEBSTER O. CRANE.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date Re. 22,150 `Bagno Aug. 4, 1942 i 1,697,126 Mayer Jan. 1, 1929 1,808,579 Sivian June 2, 1931 1,895,111 Suydam Jan. 24, 1933 2,045,172 Yungblut June 23, 1936 2,063,307 Gurtler Dec. 8, 1936 2,126,682 Hammond Aug. 9, 1938 2,235,667 Blount Mar. 18, 1941 

